“More than 19 months have passed since the murder of my fellow Texan and U.S. ICE Agent Jaime Zapata,” Cornyn wrote in a letter he sent to Holder Tuesday. “His family deserves answers, not more stonewalling. I ask that you immediately disclose the details of any ‘gun-walking’ program or tactics carried out by your Department in the State of Texas.”

Cornyn’s request came the day before another border patrol agent, Nicholas Ivie, was killed in Arizona, nearly two years after Border Patrol agent Brian Terry was shot and killed. At Terry’s crime scene, guns from the “Fast and Furious” gun-running operation were found, though they weren’t the murder weapon. The United States allowed guns to be sent into Mexico for cartel use in Operation Fast and Furious, but then lost track of some of the weapons.

Cornyn said he first requested information a year ago but has only received “a cursory response.”

“Eleven months have passed since I received this response, and your Department has failed to answer my questions,” Cornyn wrote. “This is unacceptable.”

Sen. Chuck GrassleyCharles (Chuck) Ernest GrassleyFBI informant gathered years of evidence on Russian push for US nuclear fuel deals, including Uranium One, memos show Klobuchar taking over Franken's sexual assault bill Lawyer: Kushner is 'the hero' in campaign emails regarding Russia MORE (R-Iowa) and Rep. Darrell Issa (R-Calif.) have held several hearings and asked for investigations on ATF’s gun-walking operation, which also asked Texas gun store owners to transfer weapons to suspected drug cartel agents.

“One of the weapons used to murder Agent Zapata was purchased in Texas on October 10, 2010, and was subsequently traced to a Texas-based firearms trafficking ring managed by Otilio Osorio, Ranferi Osorio, and Kelvin Morrison,” Cornyn wrote. “Even more troubling, Sen. Grassley and Rep. Issa have uncovered a trail of documents suggesting that ATF agents may have had probable cause to arrest the members of this gun trafficking ring approximately three weeks prior to the purchase of the weapon used to murder Agent Zapata. 

“At the very least, documents show that ATF could have arrested these criminals three months prior to Agent Zapata’s murder, when they illegally transferred 40 weapons to an ATF informant. They were not arrested until after Agent Zapata’s death.”